1. Field of the Invention
The present invention relates to an apparatus for controlling a variety of different mechanical devices.
2. Description of the Related Art
One example of the control apparatuses of the above type is a proportional-integral-differential (hereinafter referred to as a PID) process control apparatus. The PID control apparatus is probably the most widely used conventional machine control device since it can be applied to a variety of different mechanical devices.
FIG. 1 is a block diagram illustrating a conventional PID control apparatus. In the conventional apparatus, a reference signal S of a control signal of an object element 2 is supplied to a reference input element 4. The reference input element 4 supplies a reference input signal x.sub.s to a noninverting input terminal of a subtracter 6. The control signal x output from the object element 2 is supplied to an inverting input terminal of the subtracter 2 through a feedback element 8, as a feedback signal x.sub.f. The subtracter 2 supplies a deviation signal x.sub.s -x.sub.f to a PID control element 10. The PID control element 10 calculates an operating signal y in accordance with the known PID algorithm using the deviation signal. The control signal x of the object element 10 is controlled by the operating signal y. The object element 2 is also supplied with a disturbance signal g.
The PID control is suitable for maintaining a constant value. Therefore, the PID control is frequently used as a method of process control.
In recent years, due to the rapid development in electronics, the PID control apparatus has been developed as an auto-tuning digital PID control apparatus. The auto-tuning digital PID control apparatus can automatically set the PID constants (proportional gain, integration time, and differentiating time) in accordance with the control characteristics of the control target object and can control the control target object with a very high accuracy.
However, it has been shown that such a sophisticated control apparatus is not suitable as a means of follow-up control in which the reference value of the control signal is continuously changed. This is because that the PID control apparatus uses as its input data source only the deviation signal x.sub.s -x.sub.f and does not therefore incorporate any technique for modification of the amplitude of the proportional gain in order to prevent overshoot and hunting. Therefore, overshoot and hunting of the control signal may occur during the PID control operation due to this limitation.
Furthermore, sharp variations in the reference signal and the disturbance correspond to a frequency response in the high frequency range. Therefore, it is necessary to provide for gain and phase adjustments. These measures must be designed for the respective control apparatuses. Therefore, it is not possible to commonly apply the same control apparatus to a variety of objects.